The frequency-controlled a.c. motor is the most advanced design, for instance, for use in elevators. With frequency control, the efficiency is high at all motor speeds, and the mains power factor is nearly unity. Frequency control is appropriate for both geared and gearless elevators, and at all speeds. Moreover, a simple and moderately priced squirrel cage motor can be used for the motor. In elevator use, a transistor inverter implemented with transistors is best suited for frequency control because with transistors it is possible to achieve the highest switching frequency among existing power electronics components. GTO thyristors may also be contemplated because they have approximately equal switching times, but main current switching is more complicated than with transistors because of the required switch protection.
If pulse width modulation is applied in the inverter with a comparator in sinusoidal and triangle voltage comparison without feedback, the current which the inverter supplies to the motor is not sufficiently sinusoidal, for instance, for elevator use because in the rectified intermediate voltage circuit, whence the three-phase a.c. voltage supplying the motor is obtained by rectification with the inverter, the voltage is not constant, and also for the reason that a semiconductor switch does not follow the control without delay. A third factor causing an error is the differential voltage which is caused by the voltage existing across the power electronics component used as semiconductor switch, compared with the voltage occurring with the other direction of current when the diode which lies in parallel with the semiconductor switch is conducting. In practice, the errors give rise to vibration in the motor, which for instance in elevator operation impairs the performance of the elevator and causes discomfort to the passengers.
Presently, current feedback is a procedure known in the state of the art in inverter technology as a method for improving the curve shape. The drawback of current feedback is slow response. This is caused by the fact that, in current feedback, the motor inductances introduce time constants. The control loop is as a rule the slower the greater the number of time constants. Moreover, current measuring elements are expensive because they are also required to be able to measure direct current.